High efficiency amplification

ABSTRACT

A radio frequency amplification stage including an amplifier for receiving an input signal to be amplified and a power supply voltage; and a power supply voltage stage for supplying said power supply voltage, including a circuit for providing a reference signal representing the envelope of the input signal; a circuit for selecting one of a plurality of supply voltage levels in dependence on the reference signal; and a circuit for generating an adjusted selected power supply voltage, including an ac amplifier for amplifying a difference between the reference signal and one of the selected supply voltage level or the adjusted selected supply voltage level, and a summer for summing the amplified difference with the selected supply voltage to thereby generate the adjusted supply voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a National Stage filing under 35 U.S.C. 371of International Application No. PCT/GB2003/004507 filed 17 Oct. 2003which claims priority to GB Application No. 0303826.2 filed 19 Feb.2003; the entire disclosures of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to the control of a supply voltage, in anarrangement in which the supply voltage is selectable. The invention isparticularly but not exclusively concerned with the control of a supplyvoltage to an amplifier such as a broadband radio frequency (RF)amplifier having a wide dynamic range.

BACKGROUND TO THE INVENTION

Transistor amplifiers have a peak efficiency for a particular inputpower that is a function of geometry (i.e. circuit components andlayout), load and supply voltage. In conventional radio frequency (RF)power amplification these characteristics are fixed based on the peakinput level expected. For amplifiers presented with an input signalhaving a wide dynamic range, the input signal infrequently achieves peaklevels and frequently operates below peak levels. As such, the amplifiermay exhibit low overall efficiency.

A solution to the problem of low amplifier efficiency is to vary one ormore of the above-stated characteristics (geometry, load, supplyvoltage) in response to the input signal. Techniques to vary one or moreof these characteristics are known in the art.

Techniques that vary the device geometry and load tend to be verydependent on the particular power amplifier topology used, and generallypresent challenging RF problems. Repeatability of such designs inproduction is generally a problem.

Various techniques are known in the art for enhancing amplifierefficiency based on the supply voltage. Of supply voltage basedefficiency enhancement schemes, there are two broad classifications ofsolution. These solutions are:

-   -   (i) envelope elimination and restoration, and    -   (ii) envelope tracking.

Envelope elimination and restoration requires the amplifier to be drivensaturated, and all the envelope information to be applied through theamplifier supply. This technique tends to be generally too demandingupon the supply modulator when using high modulation bandwidths, andthus has limited usefulness in practical applications.

With envelope tracking, the amplifier is driven in a substantiallylinear fashion. Envelope tracking requires an efficient power supplycapable of delivering high modulation power bandwidths. In knowntechniques, a switched mode pulse width modulator (commonly referred toas class S) is used to realise an efficient variable supply to the poweramplifier. However, in order to operate at full bandwidth, the supplymust switch at many times the bandwidth of the modulation, and thisexcessively high switching speed results in poor modulator efficiency.

In another prior art envelope tracking technique, a plurality of highlyefficient intermediate power supplies are provided, and the powersupplies are switched as required by the envelope level. This switchingcreates transient disturbances that degrade the spectrum with high orderintermodulation products, and makes linearization difficult byintroducing supply dependent non-linearities alongside input dependentnon-linearities.

In a further modification to this technique, the switching of the powersupplies is combined with a linear amplifier to provide a smoothtransition between switch levels and remove the supply dependentlinearization requirement. The aim of this form of envelope tracking isto provide a unique value of supply voltage for every envelope level.However, there is a problem in achieving this without impact upontracking speed capability.

It is an aim of the present invention to provide an improved supplyvoltage based efficiency enhancement scheme, which preferably addressesone or more of the above-stated problems.

SUMMARY OF THE INVENTION

According to the present invention there is provide a power supplystage, comprising: reference means for providing a reference signalrepresenting a desired power supply voltage; election means forselecting one of a plurality of power supply voltages in dependence onthe reference signal; adjusting means for receiving the selected powersupply voltage and the reference signal and adapted to generate anadjusted selected power supply voltage tracking the reference signal independence thereon.

The power supply stage may be for an amplifier, the reference signalrepresenting the envelope of an input signal of said amplifier.

The adjusting means may include an AC amplifier. The selected powersupply voltage may have the minimum absolute difference between saidpower supply voltage and the reference signal level.

The ac amplifier may be connected to amplify the difference between thereference signal and a representation of the selected power supplyvoltage. The adjusting means may include means for summing the amplifieddifference with the selected supply voltage.

The representation of the selected power supply voltage may be the powersupply voltage itself. The representation of the selected power supplyvoltage may be the adjusted selected power supply voltage. The adjustedselected supply voltage may be the output of the power supply stage.

The adjusting means may further include a high frequency amplifier. Thehigh frequency amplifier may be connected to amplify the differencebetween the reference signal and a representation of the adjusted powersupply voltage. The adjusting means may include means for summing theamplified difference with the adjusted supply voltage to generate afurther adjusted supply voltage. The representation of the adjustedsupply voltage may be the adjusted supply voltage itself. Therepresentation of the adjusted supply voltage may be the furtheradjusted supply voltage. The further adjusted supply voltage may formthe output of the power supply stage.

There may further be provided an interpolation means at the input to theadjusting means such that the selected power supply voltage isinterpolated. The interpolation means comprises an inductor-capacitorarrangement.

There may further be provided means for DC clamping the AC amplifier.The means for DC clamping may be responsive to detection of thereference signal being less than the adjusted supply voltage.

There may be provided a delay element for delaying the reference signal.There may further be provided slow DC adjustment means for removing a DCcomponent from the AC amplifier.

The adjusting means may comprise a plurality of cascaded correctioncircuits. The adjusting means may comprise two or more cascadedcorrection circuits.

There may be provided a delay element for compensating for differencesin delays between the signal amplifier and power supply control input.

In a further aspect the present invention provides a radio frequencyamplification stage comprising: an amplifier for receiving an inputsignal to be amplified and a power supply voltage; and a power supplyvoltage stage for supplying said power supply voltage, comprising: meansfor providing a reference signal representing the envelope of the inputsignal; means for selecting one of a plurality of supply voltage levelsin dependence on the reference signal; and means for generating anadjusted selected power supply voltage, comprising an AC amplifier foramplifying a difference between the reference signal and one of theselected supply voltage level or the adjusted selected supply voltagelevel, and a summer for summing the amplified difference with theselected supply voltage to thereby generate the adjusted supply voltage.

The means for generating an adjusted selected supply voltage may furthergenerate a further adjusted supply voltage and further comprises an RFamplifier for amplifying a difference between the reference signal andone of the adjusted supply voltage or the further adjusted supplyvoltage, and a summer for summing such amplified difference with theadjusted supply voltage to thereby generate the further adjusted supplyvoltage.

One of the adjusted supply voltage or further adjusted supply voltagemay form the supply voltage to the amplifier.

In a still further aspect the present invention provides a method ofcontrolling a power supply stage, comprising: providing a referencesignal representing a desired power supply voltage; selecting one of aplurality of power supply voltages in dependence on the referencesignal; generating an adjusted selected power supply voltage trackingthe reference signal in dependence on the selected power supply voltageand the reference signal.

The reference signal may represent the envelope of an input signal to anamplifier, the power supply stage providing a power supply to saidamplifier. The difference between the reference signal and arepresentation of the selected power supply voltage may be AC amplified.The amplified difference may be summed with the selected supply voltageto form the adjusted supply voltage. The difference between thereference signal and a representation of the adjusted power supplyvoltage may be RF amplified.

The amplified difference may be summed with the adjusted supply voltageto form a further adjusted supply voltage. The method may furthercomprise an interpolating step prior to said step of generating anadjusted supply voltage

The method may further comprise an AC amplification step.

The present invention provides a controlled voltage source formaintaining a supply current, connected in series with the supplyvoltage source, such that the same current flows through both voltagesources.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention in now described by way of example with referenceto the accompanying Figures, in which:

FIG. 1 illustrates a block diagram of an RF amplification stageembodying the concept of the present invention;

FIG. 2( a) illustrates the concept of envelope tracking supply voltagevariation, and the problems associated therewith;

FIG. 2( b) illustrates the principle of the improvements offered by thepresent invention;

FIG. 3 illustrates in more detail an implementation of the RFamplification stage of FIG. 1 including a correction path in accordancewith the present invention;

FIG. 4 illustrates an enhanced exemplary implementation of thecorrection path of FIG. 1;

FIGS. 5( a) and 5(b) illustrate alternative implementations of thecorrection path in embodiments of the present invention;

FIG. 6 illustrates an exemplary implementation of the DC control loopsof the RF amplification stage of FIG. 1;

FIG. 7 illustrates a principle of operation in a preferred embodiment ofthe invention.

FIG. 8 illustrates an exemplary implementation of time delay elements ofthe RF amplification stage of FIG. 1; and

FIG. 9 illustrates the efficiency improvement of an RF amplificationstage in accordance with the implementation of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is described herein by way of particular examplesand specifically with reference to a preferred embodiment. It will beunderstood by one skilled in the art that the invention is not limitedto the details of the specific embodiments given herein. In particularthe invention is described herein by way of reference to an RFamplification stage. However more generally the invention may apply toany arrangement where it is necessary to switch between a plurality ofvoltage supplies.

Referring to FIG. 1, there is illustrated an RF amplification stage 100in accordance with the general principles of the present invention. TheRF amplification stage 100 includes an RF amplifier 102, a supplyvoltage selection block 106, an envelope detector 104, and a supplyvoltage adjustment block 108.

The supply voltage selection block 106 receives four supply voltagesV₁-V₄ on respective input lines 132 ₁-132 ₄. The selected supply voltageis output from the supply voltage selection block 106 on line 120. TheRF amplification stage 100 receives an RF input signal RF_(IN) on line110. The envelope detector 104 has an input 114 coupled to line 110 tothereby detect the RF input signal. The envelope detector provides anoutput on line 118 to the supply voltage selection block 106 to providethe necessary information for the supply voltage selection to takeplace. In addition, and in accordance with the present invention, theenvelope detector 104 provides a second output on line 116 to the supplyvoltage adjustment block 108. Supply voltage adjustment block 108additionally receives the output of the supply voltage selection blockon line 120. The supply voltage adjustment block 108 generates anadjusted supply voltage on line 122 for the RF amplifier 102. Theadjusted supply voltage on line 122 preferably forms a feedback input tothe supply voltage adjustment block 108. As discussed hereinbelow afeedback arrangement is preferable in embodiments of the invention,although feedforward arrangements may also be used.

As discussed in further detail hereinbelow, the supply voltageadjustment block 108 operates in accordance with the principles of thepreferred embodiment of the present invention to adjust the supplyvoltage signal on line 122, in dependence upon the signal on line 116and the feedback adjusted supply voltage on line 122, to provideadjusted supply voltage V_(S) on line 122 to the RF power amplifier 102.The RF power amplifier receives as its signal input the RF input signalon line 110. The RF amplifier 102 provides on line 112 the RF outputsignal RF_(OUT).

The operation of the RF amplification stage 100 in accordance with thepresent invention will be described further hereinbelow. Howeverreference is first made to FIG. 2(a), which illustrates the concept ofenvelope tracking supply voltage, and the problems associated therewith.Referring to FIG. 2( a), there is illustrated a plot of voltage againsttime. On the voltage axis, there is illustrated four specific voltagelevels V₁-V₄ corresponding to the voltage levels provided to the supplyvoltage selection block 106 of the RF amplification stage of FIG. 1. Itshould be noted that the provision of four voltage supplies isillustrative, and the RF amplification stage may in fact be providedwith more or less voltage supplies in accordance with implementationrequirements.

Curve 202 of FIG. 2( a) illustrates the voltage envelope of the RF inputsignal to the RF amplification stage, i.e. the signal on line 110. Thedash line curve 206 illustrates the idealized voltage supply envelopefor such an RF input signal. As can be seen, the dash line curve 206tracks the RF input signal envelope 202 to provide an ideal power supplyfor the current input signal level. As such, the idealized power supplyvoltage avoids any wasted power and consequently is very efficient.

The stepped curve 204 illustrates a typical voltage supply to an RFpower amplifier based on a switch supply voltage of four levels,reflecting performance typical in prior art implementations. As theenvelope 202 of the RF input signal reaches the voltage levels V₁-V₄,the supply voltage is appropriately switched. As can therefore be seenfrom FIG. 2( a), the supply voltage 204 steps between the four supplyvoltage levels. As such, the supply voltage level to the RF amplifier isfrequently excessive. As illustrated by the hatched area 208, thestepped supply voltage implementation of the prior art is generallysignificantly less efficient than the idealized solution. The hatchedarea 208 in FIG. 2( a) represents wasted energy, corresponding to supplyvoltage levels above the idealized level and which is consequentlyunnecessary. Referring to FIG. 2( b), there is illustrated theefficiency improvement achieved in accordance with the presentinvention. The stepped curve 205 illustrates the voltage supplygenerated by the switched supply voltages. The supply voltage 205provided by the supply voltage selection block, as discussedhereinbelow, tracks above and below the envelope 202. Such a function isprovided, in the preferred embodiment, due the use of an AC amplifier inthe supply voltage adjustment block 108. The embodiments of the presentinvention result in an actual supply voltage to the amplifier which moreclosely follows the idealized supply voltage 206, resulting in improvedefficiency and a final voltage to the amplifier which is more closelyaligned to the idealized voltage 206. As will be discussed hereinbelow,in an embodiment the supply voltage selection block may provide at itsoutput a step function equivalent to function 204 in FIG. 2( a). IN suchan arrangement a DC offset is provided between at the output of thesupply voltage selection block to provide a function equivalent tofunction 205 of FIG. 2( a).

The RF amplification stage of FIG. 1, in accordance with the presentinvention and embodiments described further hereinbelow, provides animproved solution in which the switched supply voltage more closelytracks the idealized supply voltage envelope, as shown in FIG. 2( b),and minimizes wasted energy and thereby maximizes efficiency.

The RF amplification stage 100 in accordance with the present inventioncomprises an RF amplifier 102 that may be connected to one of a numberof efficiently generated DC power supplies (V₁-V₄) through the supplyvoltage selection block 106 and the supply voltage adjustment block 108.The function of the supply voltage selection block 106 and supplyvoltage adjustment block 108 is to approximately follow, as closely aspossible, the envelope of the input signal without incurring additionalenergy dissipation, as represented by hatched area 208 in FIG. 2( a).

In general, given a selection of the desired supply voltage for the RFinput signal to be amplified, the supply voltage selection block 106connects the selected supply voltage to its output on line 120. Thesupply voltage adjustment block 108 functions to apply an appropriatelyadjusted version of the supply voltage on line 120 to the RF amplifier102 in such a way as to approximate envelope tracking at the amplifiersupply, but without introducing the reduced bandwidth or reducedefficiency associated with linear devices providing the full range of RFamplifier supply voltages. This configuration allows the amplifier toachieve high efficiency at high modulation bandwidths.

Referring to FIG. 3, there is shown a preferred implementation of the RFamplification stage 100 of FIG. 1 in accordance with the preferredembodiment of the present invention. Where elements of the RFamplification stage 100 correspond to elements shown in FIG. 1 the samereference numeral is used.

As in FIG. 1, the RF amplification stage 100 includes the envelopedetector 104, the supply voltage selection block 106, the RF amplifier102, and the supply voltage adjustment block 108. In FIG. 1, theenvelope detector 104 is illustrated providing two distinct outputs onlines 118 and 116. In the embodiment of FIG. 3, the envelope detector104 provides a single output on line 340, which provides the input tothe supply voltage selection block 106 and an input to a delay element304. The output of delay element 304 on line 334 forms an input to adigital-to-analogue converter (DAC) 306, which provides an output online 332 forming an input to the supply voltage adjustment block 108equivalent to the signal on line 116 in FIG. 1. The RF amplifier 102 isfurther provided with a delay element 302 at its input, such that the RFinput signal on line 110 forms an input to the delay element 302, andthe output of the delay element on line 346 forms the input to the RFamplifier 102. Note that in an actual implementation, delay 302 couldalternatively be connected in the path between sample 114 and envelopedetector 104 instead of before the RF amplifier. The delay element ispreferably placed in the path (supply or RF amplifier) that has thesmallest delay. The supply voltage adjustment block 108 generallyincludes an idler 310, a first feedback circuit 342 and a secondfeedback circuit 344. The idler 310 receives the output of the supplyvoltage selection block 106 on line 120. The idler 310 provides anoutput on line 324 forming a first input to a summer 314 of the firstfeedback circuit 342. The output of the summer 314 is provided on line326, and forms the output of the first feedback circuit 342. The signalon line 326 additionally forms a first input to a subtractor 316. Thesecond input of the subtractor 316 is provided by the output of the DAC306 on line 332. The output of the subtractor 316 is provided on line328 and forms an input to an AC amplifier 312, the output of which online 330 forms the second input to the summer 314. The second feedbackcircuit 344 is constructed similarly to the first feedback circuit 342.The input to the second feedback circuit 344 is provided by the outputof the first feedback circuit 342 on line 326, which forms a first inputto a summer 318 of the second feedback circuit. The output of the summer318 on line 348 forms a first input to a subtractor 322, the secondinput to which is formed by the output of the DAC 306 on line 332. Theoutput of the subtractor 322 on line 336 forms an input to a HFamplifier 320, the output of which on line 338 forms the second input ofthe summer 318. The output of the summer 318 on line 348 also forms thesupply voltage input V_(S) on line 122 to the RF amplifier 102.

In general, therefore, each feedback circuit receives an input voltageand outputs an adjusted version of said voltage. The adjustment isperformed in dependence on the output of the feedback circuit (definingthe feedback path) and a reference signal which in the describedembodiment is formed by the output of the envelope detector 104.

The supply voltage adjustment block 108 operates to provide an adjustedvoltage level to the RF amplifier 102 which tracks the voltage level ofthe envelope representing the input to the RF amplifier, but which has alarge output current corresponding to the large output current providedby the output of the supply voltage selection block 106.

The general principle of operation of the RF amplification stage 100 ofFIG. 3 is now described. The envelope detector 104 and supply voltageselection 106 provides a minimized loss predictive feed forward paththat approximates the required voltage supply tracking waveform. Theenvelope detector 104 provides on its output line 340 the detectedenvelope of the RF input signal on line 110, which information isprovided to the voltage supply selection block 106.

The supply voltage selection block 106, which is preferably implementedas a splicer network, is connected to a plurality, in the embodimentfour, of fixed high efficiency DC power supplies. The splicer network ispreferably a network of transistors and diodes specifically designed toconnect a selected supply voltage to the output on line 120 with aminimum of switching transients. The supply voltage selection 106preferably operates on the output of the envelope detector 104 to switchbetween the voltage supplies at its input as the voltage level indicatedby the envelope detector 104 of the RF input signal rises and fallsabove and below threshold voltage levels corresponding to the respectivesupply voltages. In this way, the supply voltage selection block 106provides on its output 120 an appropriate one of the supply voltagesV₁-V₄.

The implementation of the supply voltage selection block to provide avoltage function 205 as illustrated in FIG. 2( b) is within the scope ofone skilled in the art. In a preferred implementation a DSP providesdigital signals in a ‘thermometer code’ format (i.e. 0000, 0001, 0011,0111, 1111). The splicer network is preferably an arrangement of diodesand transistors that respond to this digitally coded signal by switchingto the highest voltage level representing a level ‘1’ logic signal. Alllower level switches are turned off by means of the circuit functionimplemented within the splicer network, and without intervention from aDSP.

As discussed above, in an alternative arrangement the supply voltageselections block 106 may output a function such as function 204 in FIG.2( a), and a dc offset circuit may be provided at its output. The needto have a function such as function 205 in FIG. 2( b) will becomeapparent in the following description.

The purpose of the delay block 304 at the input to the DAC 306 isdescribed further hereinbelow. Generally the delay block ensures thetiming of signals in the amplification stage is synchronized.

The DAC 306 is provided on the basis that the RF input signal is adigital signal. However, the present invention is not limited in its useto digital applications. The DAC 306 converts the envelope detectoroutput on line 340 provided via the delay element 304 into an analogueformat for input on line 332 to the supply voltage adjustment block 108.

The idler 310, and the first and second feedback circuits 342 and 344,operate on the selected supply voltage on line 120 to provide animproved and efficient voltage supply signal on line 122 to the RFamplifier.

The idler 310 provides interpolation of the signal at its input on line120 to generate an output on line 324. The idler 310 is preferably acircuit consisting of low-loss energy storage elements, and acts toredistribute energy supplied via the splicer network of the supplyvoltage selection block 106 in an optimal fashion to improve amplifierefficiency and reduce unwanted emissions.

The first feedback circuit 342 is a corrective AC feedback circuit orcorrection circuit and functions to provide fine correction to theoutput of the predictive feedforward path on line 324. The AC feedbackcircuit 342 increases efficiency

The second feedback circuit is a low power high frequency correctivecircuit which removes any residual high frequency glitches from theoutput of the first feedback circuit.

It should be noted that although the preferred embodiment of FIG. 3illustrates two feedback circuits connected in a cascade arrangement,the invention is not limited to such an arrangement. A single feedbackcircuit or any number greater than two may be provided may be provided.Where a single feedback circuit is provided, it is preferably an ACfeedback circuit. It should also be noted that the idler 310 is notessential to the present invention, and is used in preferredembodiments. Further, the supply voltage adjustment block may beimplemented by using feedforward circuit(s) in some or all of thecorrective blocks in the cascade.

In general the supply voltage selection block 106 and the envelopedetector 104, can be considered to form a minimal loss predictivefeedforward path, which provides an approximation of the requiredtracking waveform for the supply voltage to the RF amplifier. When used,the output of the idler 310 forms the output of the feedforward path.

In general, in the preferred embodiment the supply voltage adjustmentblock 108 comprises a corrective feedback path. Of course, whereprovided, the idler 310 does not form part of such path. The supplyvoltage adjustment block 108 may comprise a plurality of correctivefeedback paths. Where a plurality of corrective feedback paths areprovided, they are preferably provided in cascade. As discussed furtherhereinbelow, in embodiments preferably clamping and DC restoration meansare provided in the supply voltage adjustment block 108 in order toovercome any limitations of AC feedback and DC tracking.

The delay block 304 is preferably provided in order to remove anydifferential time delay between the predictive and corrective parts ofthe RF amplification stage 100. The delay element 302 is inserted in theRF input signal path for the purpose of compensating for the delay ofthe amplification stage 100.

The operation of the supply voltage adjustment block 108 is now furtherdescribed. The DAC 306 effectively provides on line 332 a referencesignal for use by the supply voltage adjustment block 108. The signal online 332 represents the envelope of the RF input signal detected by theenvelope detector 104, and therefore it represents a reference level forthe supply voltage which should be supplied to the RF amplifier 102 atany instant in order to achieve maximum efficiency. This envelopecorresponds to the envelope 202 of FIG. 2( b) above.

The signal provided by the supply voltage selection block 106 on line120 corresponds to the step function 205 of FIG. 2( b) above. Inaccordance with the preferred embodiment of the present invention thepower supply voltage selection block rounds to the nearest supplyvoltage level, as shown in FIG. 2( b), whereas the prior art alwaystruncates upwards as shown by curve 204 in FIG. 2( a). The purpose ofthe RF amplification stage 100 is to adapt the step function on line 120in dependence upon the reference signal on line 332 in order to providea signal at the supply voltage input to the RF amplifier which closelyapproximates the idealized supply voltage indicated by dash line 206 ofFIG. 2( b).

As discussed above, the idler 310 is not essential to the implementationof the present invention. The idler is preferably a reactive LC(inductor-capacitor) combination, provided on the output of the supplyvoltage selection block 106 in order to improve efficiency. The additionof the idler, which can store energy from the supply voltage selectionblock and release it as required by the amplifier, improves theefficiency of the RF amplification stage in achieving a closer match ofthe amplifier's optimal power supply requirements. The idler circuittherefore assists in improving efficiency for rapidly changing waveformsand for reducing unwanted emissions.

The use of the idler is advantageous, in smoothing the output of thesupply voltage selection block. It should be noted, however, thatforming a feedback loop from the output of the idler to the supplyvoltage selection block in attempt to provide correction equivalent tothe supply voltage adjustment block 108 would not provide a practicalsolution. Whilst the idler advantageously smoothes out output of thesupply voltage selection block, as its name suggests it does so at theexpense of speed. The idler is too slow to be incorporated in acorrective feedback loop, and a practical correction circuit could notbe achieved in this way.

The provision of a feedback loop in the supply voltage adjustment block,in accordance with the preferred embodiment of the invention, allows fora fast corrective loop with a wide bandwidth. Similar advantages areobtained for the feedforward case.

The AC corrective amplifier 312 of the first feedback circuit 342requires an AC signal at its input. This is achieved by the supplyvoltage selection block rounding to the nearest supply voltage levelrather than truncated to the highest level; such a step function isnecessary for the AC correction to work. As can be seen from FIG. 2( b),the input to the AC amplifier 312 is the difference between the supplyvoltage signal and the reference signal. This can only be an AC signalif the supply voltage signal alternates above and below the referencesignal, thus producing a usable input for the amplifier 312.

The process of rounding the power supplies in the voltage selectionblock 106, rather than truncating them as in the prior art, isequivalent to doubling the effective number of power supplies comparedto a prior art system.

This doubling is effectively achieved by halving the error relative tothe prior art. So for the same number of power supplies provided, withthe invention the error is halved such that there are in effect twice asmany power supplies as in the prior art. Alternatively, the inventionmay be considered to offer the same error as a prior art system withonly half the number of supplies needed.

The subtractor 316 of the first feedback circuit receives, in effect,the current output of the supply voltage selection block 106, and theenvelope reference signal on line 332. The output of the subtractor 316on line 328 is thus the difference between the two signals, representingthe error in the signal at the output of the supply voltage selectionblock compared to the ideal voltage supply 332. The amplified errorsignal on line 330 is then added to the output of the supply voltageselection block 106, in order to provide a supply voltage signal on line326 which is compensated for the error. The AC amplifier 312 has tohandle error signals with a high peak to mean ratio, and is thuspreferably implemented as a class G amplifier with multiple switchsupplies for optimum efficiencies. The amplifier 312 handles much lowerpower levels than the supply voltage selection block 106, and thereforecan be implemented with smaller, faster devices.

The second feedback circuit 344 provides for additional correctionmeans. Further correction means may be added as required, andfurthermore only one correction means may be required. The correctivecircuits of the supply voltage adjustment block 108 effectively forcethe supply voltage of the RF amplifier to closely track the referenceprovided by the envelope detector 104 over a wide range of frequencies.

The summation means 314 and 318 may preferably be implemented astransformers.

The supply voltage selection block 106, preferably comprising a splicerincluding a switch matrix, is preferably controlled by a digitalsignaling processing means, which digital signaling processing means mayfurther include the delay elements 302 and 304, the DAC 306 and theenvelope detector 104. The specific implementation of such digitalsignaling processing means, including the various associated elements,will be within the scope of a person skilled in the art.

Implementing the AC amplifier 312 of the first feedback circuit 342 witha plurality of switched voltage supplies advantageously means that thenumber of supply voltages to the RF amplifier 102 may be significantlyincreased. For example, if the AC amplifier 312 was associated with msupply voltages, and the main supply voltage selection is associatedwith n supply voltages, the total number of supply voltages available tothe RF amplifier 102 is m*n. As discussed above, the use of power supplyrounding in the supply voltage selection block effectively doubles thenumber of power supplies, such that the total number of power suppliedmay become 2*m*n.

Such an embodiment of the present invention offers significantadvantages. As discussed above, the small size of the amplifier 312means that it is not associated with the problems associated with thesupply voltage selection block 106, which problems the invention isintended to overcome. The envelope tracking feedback loop of the supplyvoltage adjustment block 108 does not require any large geometrydevices, or the high current large geometry devices being providing atthe predictive feedforward path rather than the feedback loop. As such,there is provided a large increase in the power bandwidth of theenvelope tracking loop.

Referring to FIG. 4, there is illustrated the adaptation of theamplifier 312 in order to provide a variable supply voltage to saidamplifier. The provision of such variable supply to the amplifier may bein accordance with techniques known in the art. As shown in FIG. 4,there is provided a voltage selector 402, which receives an input of aline 408 coupled to the signal input of the amplifier. The input on line408 represents the amplitude of the input signal. The voltage selector402 also receives a plurality of supply voltages, e.g. two supplyvoltages Vx and Vy on lines 406 and 404. The voltage selector 402selects one of the supply voltages for output on line 410 as the supplyvoltage for the amplifier. The supply voltage Vx and Vy is selected independence on the envelope on line 408.

The implementation of the amplifier 312 with selectable power suppliesis not limited to such an arrangement. The implementation may beachieved, for example, under the control of a DSP.

As discussed further hereinbelow, in a preferred embodiment the use ofclamping and restoration means removes the need for any DC correctivefeedback.

Further discussion of detailed, preferable aspects of implementations ofthe present invention are discussed hereinbelow. Firstly, theimplementation of an embodiment of the present invention utilizing afeedforward corrective circuit is discussed.

In the embodiments of FIGS. 1 and 3, the corrective circuit orcorrective path in the supply voltage adjustment block is implemented asa feedback arrangement. The general principle of such an arrangement isshown in FIG. 5( a). Generally an approximate function block 556generates a signal to be corrected, and a reference function block 554generates a reference signal. The signal to be corrected forms a firstinput to a summer 552, the output of which on line 558 represents thecorrected signal. The reference signal and the corrected signal forminputs to an amplifier 550, the output of which forms a second input tothe summer 552. It should be noted that this is the general principle ofthe feedback paths of FIG. 3, the input to the amplifier 312 beingprovided by a subtractor which receives the corrected output and thereference signal as inputs. Referring to FIG. 5( b), there isillustrated the principle, in an alternative embodiment, of afeedforward arrangement. The signal to be corrected again forms a firstinput to the summer 562. An amplifier 560 receives as inputs the signalto be corrected and the reference signal. The amplifier output forms thesecond input to the summer 562, the output of which again forms thecorrected output. A feedforward arrangement as shown in FIG. 5( b) maybe used in the embodiments of FIGS. 1 and 3 in place of a feedbackarrangement.

As the splicer network of the supply voltage selection block isimplemented as part of a predictive feedforward controller, adaptivecontrol of the splicer parameters improves efficiency. A local versionof the splicer output is therefore preferably generated digitally frommodels of the elements in the splicer network. The decision to changethe splicer level, i.e. the output supply voltage, is made in order tominimize the difference between the locally generated splicer waveform(including consideration of past, present and future values) and theenvelope representation. Included in this model are:

-   -   a) switch delay and risetime parameters;    -   b) filter parameters; and    -   c) splicer voltages and FET resistances.

The locally generated parameters are related to the actual parameters bycomparing locally generated measured parameters with those obtained fromthe actual circuit. The actual parameters that are most convenient tomeasure is the signal error from comparison of the splicer output andthe averaged current consumption of the AC error amplifier 312 of thefirst feedback circuit 342. An optimization algorithm may be providedwhich aims to minimize the current consumption of the AC amplifier 312by adjustment of the above measured parameters in the model. Convergenceof the optimization will be assisted by consideration of the signederror of the splicer network to provide localized error correction.Selection of the supply to which the amplifier will be connected is madeby a supply selection circuit.

The splicer circuit of the supply voltage selection block 106 consistsof a network of switching transistors that are switched by logic signalsoriginating in the digital signal processing (DSP) means. Theimplementation of such adaptive control will be understood by oneskilled in the art.

Referring to FIG. 6, there is illustrated the implementation of DCclamping and dc offset restoration in accordance with a preferredembodiment of the invention. DC clamping is provided to preventaccumulation of a DC error signal. DC offset restoration compensates forany adjustment in the actual supply voltage level. For example, althoughthe selected supply voltage is 7V, it may actually be only 6.5V due tocircuit degradations. DC offset restoration and DC clamping may beprovided together, as described in this embodiment, or separately.

As before, elements in FIG. 6 corresponding to elements in earlierfigures are identified by the same reference numerals. FIG. 6illustrates only a portion of the RF amplification stage 100. FIG. 6illustrates the supply voltage selection block 106, and the firstfeedback circuit or correction circuit 342 as illustrated in FIG. 3.Also shown is the DAC 306 of FIG. 3. In FIG. 6, the DAC 306 is shown toreceive an input on lines 504 from a DSP 500. The DSP 500 also providescontrol inputs on lines 502 to the supply voltage selection block 106.As discussed hereinabove, in such an embodiment the DSP 500 may beconsidered to include the envelope detector 104 and delay element 304 ofFIG. 3. As further shown in FIG. 6, the output of the first feedbackcircuit or correction circuit 342 on line 326 is provided to either afurther cascaded feedback/correction circuit or directly to the supplyvoltage of the RF amplifier. In order to accommodate DC clamping, the RFamplification stage is further modified to include a subtractor 512,which receives as a first input the output on line 120 of the firstfeedback/correction circuit, and as a second input the reference signalon line 332 from the DAC 306. The output of the subtractor 512 isprovided on line 510 to a single bit analogue-to-digital converter 508,which provides an output on line 506 to the DSP 500. The DSP 500receives as an input the output of the subtractor 316, representing theerror signal in the supply voltage signal. In addition the DSP 500generates an output on line 518 which forms an input to a V_(DD) clampcircuit 514. The output of the clamp circuit 514 on line 516 controls aclamp means, generally designated by reference numeral 520, connected tothe output 330 of the AC amplifier 312.

Referring to FIG. 7, it can be understood that the envelope signal goesbelow the lowest supply voltage level at certain points. For example, ifit is assumed for the sake of example that line 804 c represents alowest supply voltage, between points 803 and 805 the envelope goesbeneath the lowest supply voltage. During such time interval, there isno need for the supply voltage to the amplifier to be tracked by thecorrection circuit. The embodiments of the present invention thereforedetect this condition, and use the existence of the condition to applydc restoration as discussed further hereinbelow.

The subtractor 512 subtracts the reference signal on line 332 from thecurrent supply voltage on line 120, and provides the difference to the 1bit analogue to digital converter 508. The most significant bitidentifies whether the current envelope level is above or below thelowest supply voltage fine control of envelope clamp off level beingachieved by integration over time.

Responsive to the DSP detecting the appropriate condition, a commandsignal is sent to the V_(DD) clamp circuit on line 518, which in turncontrols the clamp means 50 to turn on. When turned on the clamp means520 pulls the output of the ac amplifier on line 330 to a knownreference level, preferably ground. The clamp means 520 is preferably atransistor.

The purpose of the DC clamping is to reset the amplifier 312 to a knownstate in order to avoid the build-up, over time, of a DC error. This isachieved as described above in a preferred embodiment. However othertechniques may be provided for dc clamping. For example, the dc may befed back, for example, by slow adjustment of the plurality of powersupplies to the selection block 106 or use of a series pass transistor.Thus a certain time period is allocated as the DC clamping interval.

This comparison is necessary to allow for voltage drops in anycomponents and conductors connected between the current supply voltageand the lower supply voltage during the clamp period. In any practicalimplementation, there will always be some uncertainty between the actuallower supply voltage level when delivering current to the RF amplifierand the level initially assumed by the clamped level at the envelopedetector. The comparator 512 corrects for this and ensures that theenvelope detector is clamped off at a level equal to the actual lowersupply voltage.

The signal on line 506 is preferably gated in an integrator in the DSP500. The output of the integrator sets the clamp level of the amplitudereference digital output on line 518. The effect is to alter the clamplevel of the reference DAC 306. The integration thus compensates forvariations in the supply voltage such that clamp-off occurs at theactual supply voltage (e.g. 6.5V) rather than the theoretical supplyvoltage (e.g. 7V). The integrator thus adjusts the clamping such thatthe clamp off voltage is equal to the value of the lowest supply inputto 106 minus any additional voltage drops in 106.

During the clamping period, the value of the AC feedback output is setto zero by switches 520 that are turned on during the clamping period.This DC restoration enables an amplifier to be used with a finite lowercut-off frequency.

The error waveform has a flat frequency spectrum, which implies that theuse of a bandpass AC amplifier, such as amplifier 312, may result insignificant residual error due to the inability of the amplifier tofollow low frequency components. By clamping the AC output during theclamping interval, and provided the low frequency time constant of theamplifier 312 is more than the interval between clamps, accuratetracking can be assured.

Referring to FIG. 7, the advantage achieved in the preferred embodimentutilizing DC clamping is illustrated. The curve 802 of FIG. 7 representsthe envelope of the RF input signal. The straight lines 804 a-804 billustrate the adjustment of the clamped DAC levels, i.e. the effect ofdc removal.

Another significant source of low frequency error is any mismatchbetween the expected splicing voltages in the DSP 500 and the voltagesexisting at the output of the supply voltage selection block 106. Thisis equivalent to an error in the slicing threshold. A splicer adaptationalgorithm, as described above, removes such errors in the thresholding.

Another DSP adjustment which may be preferably implemented is to delaythe DAC envelope reference waveform output relative to the output of thesupply voltage selection block 106, as represented by delay element 304in FIG. 3 and discussed hereinabove.

An example delay balancing implementation is illustrated in FIG. 8,which includes the modifications introduced in FIG. 6 to provide for aclamping of a DC level in the feedback/correction circuit. The provisionof the delay balancing implementation is preferable so as to affectmaximum efficiency, since the AC amplifier 312 output is then minimized.An appropriate modification to the preferred embodiment of FIG. 6 isshown in the further preferred embodiment of FIG. 8. Only those elementsnecessary to understand this embodiment of the invention areillustrated.

Referring to FIG. 8, the digital signal processor 500 includes, in thispreferred embodiment, an interpolator 602, a time differentiator 604, amultiplier 608, a summer 606, a splice control block 610, and anenvelope level source 612. As will be understood, the envelope levelsource 612 represents the generation of the reference envelope, thepreferable generation of which has been described hereinabove withreference to FIGS. 1 and 3.

In accordance with this embodiment of the invention, the elements of theDSP 500 shown in FIG. 8 function to provide delay balancing.

The envelope reference waveform generated by the envelope level block612 on line 614 is provided to the interpolator 602, which provides anoutput on line 616 to the time differentiator 604. The output on line616 also forms the input to the DAC 306 on line 332. The output of thetime differentiator 604 on line 622 comprises the time derivative of theenvelope reference waveform.

The time derivative of the envelope reference waveform on line 622 ismultiplied in multiplier 608 with the single bit error signal on line506 from the one-bit ADC 508. The multiplied result is presented on line620 at the output of the multiplier 608.

The multiplied result on line 620 forms a first input to the summer 606,which is configured to function as an integrator. The output of thesummer 606 on line 618 is fed back to form a second input to the summer.The integrated output on line 618 is provided as an input to theinterpolator 602 to control or steer the delay of the referencewaveform.

Sub-sample interval delay resolution can be achieved by interpolation ofthe reference waveform in the interpolator 602.

In summary, FIG. 8 shows a preferred implementation of the delay element304 of FIG. 3, including the dc clamping control of FIG. 6. Otherpossible implementations of the delay element 304 will be within thescope of one skilled in the art.

The embodiments described hereinabove allow the RF amplification stageto operate without external adjustment for the majority of multi-carriercases within power and bandwidth constraints. However there are twospecific cases which may require the DSP 500 of FIG. 8 to automaticallymodify its operating mode. Some modification would be required to enablethe envelope tracking to be accurate for constant envelope or slowlyvarying signals. An example of such a signal is GPRS (general packetradio services) in which only one carrier or two closely spaced carriersare required to be amplified. In such a scenario, the interval betweensuccessive clamps could be very long. This would require the DSP toevoke a time-out at which point second order delta sigma pulse widthmodulation between the nearest supplies is carried out by the splicecontrol module 610. The use of pulse width modulation is acceptable insuch a scenario because there is minimal high frequency envelope powerpresent.

A further scenario arises when two carriers of an equal amplitude arepresent but separated by a large frequency interval, but the envelopedepth is not sufficient to reach the minimum supply clamp. In this case,the clamping level is moved to the next highest supply level, and allother functions remain as in the multi-carrier case.

Finally, referring to FIG. 9, there is illustrated the efficiencyimprovement provided in accordance with the present invention. FIG. 9illustrates a plot of power amplifier efficiency against power amplifiersupply voltage. As can be seen, the thick line 704 represents the supplyvoltage switching in accordance with the present invention, whereas thethin line 702 represents the range in power supply voltage achieved inthe prior art.

Curve 706 of FIG. 9 illustrates the significant advantage which can beachieved when multiple supply voltages are used including theimplementation of amplifier 312 as a Class G amplifier. As more supplyvoltages are provided, then the drop in efficiency between supplyvoltage levels is more of a ripple than the ‘sawtooth’ waveform 704achieved with a small number of power supplies.

The RF amplifier 102 preferably drives an RF load such as an antenna.

The present invention has been described herein by way of reference toparticular preferred embodiments. However the invention is not limitedto such embodiments. The present invention has p1 articular applicationin relation to RF amplifiers, but is not limited to such implementation.The invention can be advantageously utilized in any environment whereswitched, selectable voltage supplies are provided.

The described preferred embodiments utilizing an RF amplifier are notlimited to any particular load being driven by such RF amplifier.However it is envisaged that such an RF amplifier will typically drivean antenna. As such, the present invention has particularly advantageoususes in the field of communications, including the field of mobilecommunications.

1. A power supply stage, comprising: a. reference means for providing areference signal representing a desired power supply voltage; b.selection means for selecting one of a plurality of power supplyvoltages in dependence on the reference signal, the selection meansbeing adapted for selecting a power supply voltage above and below thereference signal, in dependence on the value of the reference signal; c.adjusting means adapted to generate an adjusted selected power supplyvoltage tracking the reference signal in dependence on the selectedpower supply voltage and the reference signal.
 2. A power supply stageaccording to claim 1 wherein the power supply stage is for an amplifier,the reference signal representing the envelope of an input signal ofsaid amplifier.
 3. A power supply stage according to claim 2 in whichthere is provided a delay element for compensating for differences indelays between the signal amplifier and power supply control input.
 4. Apower supply stage according to claim 1 wherein the adjusting meansincludes an AC amplifier.
 5. A power supply according to claim 4 whereinthe selected power supply voltage has a minimum absolute differencebetween said power supply voltage and the reference signal level.
 6. Apower supply stage according to claim 4 wherein the ac amplifier isconnected to amplify the difference between the reference signal and arepresentation of the selected power supply voltage.
 7. A power supplystage according to claim 6 wherein the adjusting means includes meansfor summing the amplified difference with the selected supply voltage.8. A power supply stage according to claim 7, wherein the representationof the selected power supply voltage is the power supply voltage itself.9. A power supply stage according to claim 7 wherein the representationof the selected power supply voltage is the adjusted selected powersupply voltage.
 10. A power supply stage according to claim 4 whereinthe adjusting means further includes a high frequency amplifier.
 11. Apower supply stage according to claim 10 wherein the high frequencyamplifier is connected to amplify the difference between the referencesignal and a representation of the adjusted power supply voltage.
 12. Apower supply stage according to claim 11 wherein the adjusting meansincludes means for summing the amplified difference with the adjustedsupply voltage to generate a further adjusted supply voltage.
 13. Apower supply stage according to claim 12 wherein the representation ofthe adjusted supply voltage is the adjusted supply voltage itself.
 14. Apower supply stage according to claim 12 wherein the representation ofthe adjusted supply voltage is the further adjusted supply voltage. 15.A power supply stage according to claim 12 wherein the further adjustedsupply voltage forms the output of the power supply stage.
 16. A powersupply stage according to claim 4 wherein there is further providedmeans for DC clamping the AC amplifier.
 17. A power supply stageaccording to claim 16 wherein the means for DC clamping is responsive todetection of the reference signal being less than the adjusted supplyvoltage.
 18. A power supply stage according to claim 4 wherein there isfurther provided slow DC adjustment means for removing a DC componentfrom the AC amplifier.
 19. A power supply stage according to claim 1wherein the adjusted selected supply voltage is the output of the powersupply stage.
 20. A power supply stage according to claim 1 in whichthere is further provided an interpolation means at the input to theadjusting means such that the selected power supply voltage isinterpolated.
 21. A power supply stage according to claim 20 wherein theinterpolation means comprises an inductor-capacitor arrangement.
 22. Apower supply stage according to claim 1 in which there is provided adelay element for delaying the reference signal.
 23. A power supplystage according to claim 1 in which the adjusting means comprises aplurality of cascaded correction circuits.
 24. A power supply accordingto claim 23 in which the adjusting means comprises two or more cascadedcorrection circuits.
 25. A radio frequency amplification stagecomprising: a. an amplifier for receiving an input signal to beamplified and a power supply voltage; and b. a power supply voltagestage for supplying said power supply voltage, comprising: i. means forproviding a reference signal representing a desired power supply voltagecorresponding to the envelope of the input signal; ii. means forselecting one of a plurality of supply voltage levels in dependence onthe reference signal, the means for selecting being adapted forselecting the supply voltage level above and below the reference signal;and iii. means for generating an adjusted selected power supply voltage,comprising an AC amplifier for amplifying a difference between thereference signal and one of the selected supply voltage level or theadjusted selected supply voltage level, and a summer for summing theamplified difference with the selected supply voltage to therebygenerate the adjusted supply voltage.
 26. A radio frequencyamplification stage according to claim 25 in which the means forgenerating an adjusted selected supply voltage further generates afurther adjusted supply voltage and further comprises an RF amplifierfor amplifying a difference between the reference signal and one of theadjusted supply voltage or the further adjusted supply voltage, and asummer for summing such amplified difference with the adjusted supplyvoltage to thereby generate the further adjusted supply voltage.
 27. Aradio frequency amplification stage according to claim 25 wherein one ofthe adjusted supply voltage or further adjusted supply voltage forms thesupply voltage to the amplifier.
 28. A method of controlling a powersupply stage, comprising: a. providing a reference signal representing adesired power supply voltage; b. selecting one of a plurality of powersupply voltages in dependence on the reference signal, the selectingstep being adapted for selecting a power supply voltage above and belowthe reference signal, in dependence on the value of the referencesignal; c. generating an adjusted selected power supply voltage trackingthe reference signal in dependence on the selected power supply voltageand the reference signal.
 29. A method according to claim 28 wherein thereference signal represents the envelope of an input signal to anamplifier, the power supply stage providing a power supply to saidamplifier.
 30. A method according to claim 28 wherein the differencebetween the reference signal and a representation of the selected powersupply voltage is AC amplified.
 31. A method according to claim 30wherein the amplified difference is summed with the selected supplyvoltage to form the adjusted supply voltage.
 32. A method according toclaim 31 wherein the difference between the reference signal and arepresentation of the adjusted power supply voltage is RF amplified. 33.A method according to claim 32 wherein the amplified difference issummed with the adjusted supply voltage to form a further adjustedsupply voltage.
 34. A method according to claim 30 further comprisingproviding means for removing a dc component in the AC amplificationstep.
 35. A method according to claim 28 further comprising aninterpolating step prior to said step of generating an adjusted supplyvoltage.
 36. A power supply stage, comprising: a. an envelope detectorfor providing a reference signal representing a desired power supplyvoltage; b. a supply voltage selection block for selecting one of aplurality of power supply voltages in dependence on the referencesignal, the supply voltage selection block being adapted for selecting apower supply voltage above and below the reference signal in dependenceon the value of the reference signal; c. a supply voltage adjustmentblock adapted to generate an adjusted selected power supply voltagetracking the reference signal in dependence on the selected power supplyvoltage and the reference signal.
 37. A radio frequency amplificationstage comprising: a. an amplifier for receiving an input signal to beamplified and a power supply voltage; and b. a power supply voltagestage for supplying said power supply voltage, comprising: i. anenvelope detector for providing a reference signal representing adesired power supply voltage corresponding to the envelope of the inputsignal; ii. a supply voltage selection block for selecting one of aplurality of supply voltage levels in dependence on the referencesignal, the supply voltage selection block being adapted for selecting asupply voltage level above and below the reference signal; and iii. asupply voltage adjustment block for generating an adjusted selectedpower supply voltage, comprising an AC amplifier for amplifying adifference between the reference signal and one of the selected supplyvoltage level or the adjusted selected supply voltage level, and asummer for summing the amplified difference with the selected supplyvoltage to thereby generate the adjusted supply voltage.
 38. A powersupply stage, comprising: reference means for providing a referencesignal representing a desired power supply voltage; selection means forselecting one of a plurality of power supply voltages in dependence onthe reference signal, the selection means being adapted for selecting apower supply voltage below the reference signal, in dependence on thevalue of the reference signal; and adjusting means adapted to generatean adjusted selected power supply voltage tracking the reference signalin dependence on the selected power supply voltage and the referencesignal.
 39. A power supply stage according to claim 38, the selectionmeans being adapted for selecting a power supply voltage such that thepower supply voltage tracks above and below the reference signal.
 40. Amethod of controlling a power supply stage, comprising: providing areference signal representing a desired power supply voltage; selectingone of a plurality of power supply voltages in dependence on thereference signal, the selecting step being adapted for selecting a powersupply voltage below the reference signal, in dependence on the value ofthe reference signal; and generating an adjusted selected power supplyvoltage tracking the reference signal in dependence on the selectedpower supply voltage and the reference signal.
 41. A method according toclaim 40, the selecting step being adapted for selecting a power supplyvoltage such that the power supply voltage tracks above and below thereference signal.
 42. A power supply stage, comprising: an envelopedetector for providing a reference signal representing a desired powersupply voltage; a supply voltage selection block for selecting one of aplurality of power supply voltages in dependence on the referencesignal, the supply voltage selection block being adapted for selecting apower supply voltage below the reference signal in dependence on thevalue of the reference signal; and a supply voltage adjustment blockadapted to generate an adjusted selected power supply voltage trackingthe reference signal in dependence on the selected power supply voltageand the reference signal.
 43. A power supply stage according to claim42, the selection means being adapted for selecting a power supplyvoltage such that the power supply voltage tracks above and below thereference signal.